KABUL, Afghanistan (AP) - A helicopter crash in southern Afghanistan has killed five American service members, officials said Tuesday.

Monday night's crash brought the total number of U.S. troops killed that day to seven, making it the deadliest day for U.S. forces so far this year. Two U.S. special operations forces were gunned down hours earlier in an insider attack by an Afghan policeman in eastern Afghanistan.

The NATO military coalition said in a statement that "initial reports" showed no enemy activity in the area at the time. The cause of the crash is under investigation, the statement said.

A U.S. official said all five of the dead were American. The official said the helicopter went down outside Kandahar city, the capital of Kandahar province. The official spoke on condition of anonymity because the information had not been formally released.

The five dead included everyone aboard the UH-60 Black Hawk, said Maj. Adam Wojack, a spokesman for the international military coalition in Afghanistan.

Their deaths make 12 U.S. troops killed so far this year in Afghanistan. There were 297 U.S. service members killed in Afghanistan in 2012, according to an Associated Press tally.

It was the deadliest crash since August, when a U.S. military helicopter crashed during a firefight with insurgents in a remote area of Kandahar. Seven Americans and four Afghans died in that crash.

In March 2012, a helicopter crashed near the Afghan capital, Kabul, killing 12 Turkish soldiers on board and four Afghan civilians on the ground, officials said. And in August 2011, insurgents shot down a Chinook helicopter, killing 30 American troops, mostly elite Navy SEALs, in Wardak province in central Afghanistan.

Also Tuesday, a statement from the Interior Ministry said insurgent attacks killed six Afghan civilians.

Four died when the tractor they were on struck a roadside bomb in the southern province of Helmand on Monday. Then on Tuesday, two women were killed when a mortar fired by insurgents hit their house in the same province.